Technical Field
The present disclosure relates to a light emitting device and the manufacturing method thereof, in particular to a chip-scale packaging light emitting device including a light emitting diode (LED) semiconductor die which generates electromagnetic radiation while it is in operation.
Description of the Related Art
LEDs are widely used in various applications including traffic lights, backlight units, general lightings, portable devices, automotive lighting and so forth. Generally, an LED semiconductor die is disposed inside a package structure, such as a lead frame, to form a packaged LED device. It may further be disposed and covered by photoluminescent materials, such as phosphors, to form a phosphor-converted white LED device.
Among them, plastic leaded chip carrier (PLCC) type LED devices can be divided into two categories, including top-view LED devices and side-view LED devices, according to their illumination direction. The top-view LED devices are used in general lighting or used as a backlight source in direct backlit LED TVs, while the side-view LED devices are used as a backlight source in edge-lit displays such as LED TVs or mobile phone display panels. Either top-view or side-view LED has a light-emitting surface, for example a rectangular light-emitting surface, wherein an optical axis of the LED device is specified to be a vertical axis perpendicular to the plane of the light-emitting surface and passing through the center of the light-emitting surface. In this disclosure, for the purpose of illustration, a first horizontal direction and a second horizontal direction are specified such that the first horizontal direction is perpendicular to the second horizontal direction and both of the first and the second horizontal directions are perpendicular to the vertical optical axis. The first horizontal direction is further specified to be aligned with the length direction of the LED device; and the second horizontal direction is specified to be aligned with the width direction of the LED device. It is found that if an optical radiation pattern is measured across the first horizontal direction or across the second horizontal direction of a top-view (or side-view) LED device, both radiation patterns are very similar. Since a top-view LED device or a side-view LED device typically has a similar light radiation pattern along the first or along the second horizontal direction, the PLCC-type LED device shows a symmetrical radiation pattern.
The LED devices having symmetrical radiation pattern cannot meet the needs of some applications specifying asymmetrical light sources. For example, street lighting generally specifies a radiation pattern like a “batwing” alongside the street direction. Another example is the LED light source used for a backlight unit as part of the edge-lit LED TV or the display panel for a portable electronic device, where a LED device with a rectangular light-emitting surface is desirable. More desirably, an asymmetrical radiation pattern provides a large viewing angle radiation pattern along the length direction of the LED device, which in turn is aligned with the direction of the light-guide plate for the backlight unit, so that the large viewing angle radiation pattern provides a more uniform light distribution. Therefore the edge-lit light source can reduce dark spots inside the light-guide plate, or alternatively reducing the quantity of the LED devices included. Further, this edge-lit light source is also specified to provide a small viewing angle light radiation pattern along the width direction of the LED device, which in turn is aligned with the thickness direction of the light guide, so that the incident light irradiated from the LED device can penetrate into the thin light-guide plate with a higher transmission efficiency, thus reducing light loss.
Generally, either a top-view PLCC-type LED device or a side-view PLCC-type LED device is fabricated comprising three components: a lead frame having a reflective-cup housing structure formed using a molding process, an LED semiconductor die, and a photoluminescent structure including photoluminescent materials such as phosphors. If an asymmetrical radiation pattern is specified for certain lighting applications, a typical solution is to use a PLCC-type LED device with a symmetrical radiation pattern together with a secondary optical lens to reshape the radiation pattern to achieve the desired asymmetrical light radiation pattern. This will inevitably lead to a significant increase in manufacturing cost. Extra space will be involved to accommodate the secondary optical lens as well, which is unfavorable to end product design for compact consumer electronics. If there is space constraint that the optical lens cannot be incorporated to reshape the light radiation pattern irradiated from PLCC-type LED devices, another solution is to fabricate an asymmetrical reflective cup of the lead frame. For example, two sides of the reflective cup structure is optically reflective so that the radiation pattern along this direction has a smaller viewing angle; and two sides of the cup is optically transparent so that the radiation pattern along this direction has a larger viewing angle. However, this kind asymmetrical reflective cup having two transparent sides is very difficult to fabricate in production. In other words, there remains a need for a streamlined and low-cost method to achieve an asymmetrical light radiation pattern using a PLCC-type LED device.
As the consumer electronics such as LED TVs or portable electronics devices continue to move toward thinner or more compact in size, the PLCC-type LED device as the backlight source also has to reduce its size accordingly. In this trend, the development of chip-scale packaging (CSP) LED device attracts more and more attention of the LED industry due to its small form factor. For example, the CSP LED device has been introduced to replace the widely used top-view PLCC-type LED device used in a direct back-lit LED TV, to further reduce the size of its light source. In this way, a higher light intensity is achievable, thus reducing quantity of the LED devices used. Also, a smaller form factor of the CSP LED device facilitates the design of an even smaller secondary optical lens, thus a thinner TV.
According to the illumination geometry, CSP LED devices can be categorized into two types: 1) top-surface light-emitting (top emitting) CSP LED devices and 2) five-surface light-emitting (five-surface emitting) CSP LED devices. The top emitting CSP LED device is fabricated by incorporating a reflective material disposed to cover the four peripheral edge surfaces of the LED semiconductor die, so light beam is irradiated primarily or solely from the upper surface. Therefore a top emitting CSP LED device has a similar radiation pattern as a PLCC type LED device having a smaller viewing angle (about 120°). On the other hand, light beam can be irradiated outwardly through its upper surface and four peripheral edge surfaces of the five-surface CSP LED devices. Therefore a five-surface emitting CSP LED device has a larger viewing angle (about 140°˜160°). However, similar to a PLCC-type LED device, both the top emitting CSP LED device and the five-surface emitting CSP LED device have a symmetrical light radiation pattern, thus unfavorable to applications specifying an asymmetrical light radiation pattern.
Another approach uses a CSP LED device together with a secondary optical lens to produce an asymmetrical radiation pattern. However, this approach not only significantly increases the production cost, but also involves extra space to accommodate the optical lens, thus defeating the advantage of using a small form factor CSP LED device. In other words, effective solution is still lacking for using CSP LED devices to achieve asymmetrical radiation pattern. Therefore, providing a low-cost and effective solution is desired for the CSP LED devices to achieve an asymmetrical light radiation pattern while keeping the advantage of its compact form factor.